Mandatory evacuations were ordered southeast of New Orleans on Thursday as tropical storm Barry formed in the Gulf of Mexico, threatening the city and a surrounding stretch of the Gulf with a possible hurricane with relatively weak winds but torrential rains that could send water spilling over levees.

Barry could become the first hurricane of the season by Friday, coming ashore along the Louisiana-Mississippi-Texas coastline and pouring more water into the already swollen Mississippi River. On Wednesday, the weather disturbance dumped as much as 20 centimetres in just three hours over parts of metro New Orleans, triggering flash flooding.

The U.S. National Hurricane Center said the storm’s maximum sustained winds by Thursday afternoon were 65 km/h, and forecasters said it’s likely to become a Category 1 hurricane before making landfall Friday or early Saturday. Winds could reach a speed of 120 km/h when the hurricane comes ashore.

As the storm was tracked 145 kilometres south of the Mississippi River, a hurricane warning was issued for parts of the Louisiana coast, from Intracoastal City to Grand Isle. A tropical storm warning remained in effect for other areas, including the New Orleans metro area.

Matt Harrington boards up a shoe store near the French Quarter in New Orleans on Thursday as tropical storm Barry approaches. (Seth Herald/AFP/Getty Images)

The biggest danger in the days to come is not destructive winds but ceaseless rain, the hurricane centre warned. “The slow movement of this system will result in a long duration heavy rainfall threat along the central Gulf Coast and inland through the lower Mississippi Valley through the weekend and potentially into next week.”

The storm is expected to bring more than half a metre of rain in potentially ruinous downpours that could go on for hours as the storm passes through the metropolitan area of nearly 1.3 million people and pushes slowly inland. The storm’s surge at the mouth of the Mississippi could also mean a river that’s been running high for months will rise even higher.

New Orleans got an early taste Wednesday of what may be in store. News outlets said a tornado may have been responsible for wind damage to one home, while floodwaters invaded some downtown hotels and businesses as streets became small rivers that accommodated kayakers. The floods paralyzed rush-hour traffic and stalled cars around the city.

“I must have got to work about a quarter to seven,” said Donald Smith, who saw his restaurant on Basin Street flood for the third time this year. “By 7:15, water was everywhere.”

It brought memories of a 2017 flash flood that exposed major problems — and led to major personnel changes — at the Sewerage and Water Board, which oversees street drainage. City officials said the pumping system that drains streets was at full capacity. But the immense amount of rain in three hours would overwhelm any system, said Sewerage and Water Board director Ghassan Korban.

Emergency declared, cruise ship diverted

Louisiana Gov. John Bel Edwards declared an emergency and said National Guard troops and high-water vehicles will be positioned all over the state.

“The entire coast of Louisiana is at play in this storm,” he warned, issuing a series of tweets with advice for residents.

Do NOT attempt to drive on flooded roads. And while we expect this to be a primarily rain event, it doesn’t take much wind to topple trees on already saturated grounds. <a href=”https://twitter.com/hashtag/lagov?src=hash&amp;ref_src=twsrc%5Etfw”>#lagov</a> <a href=”https://twitter.com/hashtag/lalege?src=hash&amp;ref_src=twsrc%5Etfw”>#lalege</a> <a href=”https://twitter.com/hashtag/lawx?src=hash&amp;ref_src=twsrc%5Etfw”>#lawx</a>

New Orleans officials asked people to keep at least three days of supplies on hand and to keep their neighborhood storm drains clear so water can move quickly.

A spokesperson for the Army Corps of Engineers in New Orleans said the agency was not expecting widespread overtopping of the levees, but there are concerns for areas south of the city. The weather service expects the river to rise to six metres by Saturday morning at a key gauge in the New Orleans area, which is protected by levees six to 7.6 metres high.

People in the Broadmoor neighbourhood in New Orleans cope with the aftermath of Wednesday’s severe weather. (Nick Reimann/The Advocate via The Associated Press)

Plaquemines Parish made sandbags available to people in areas not under evacuations orders, as did several other communities in Louisiana and Mississippi.

The Army Corps of Engineers was working with local officials down river to identify any low-lying areas and reinforce them, he said. He cautioned the situation may change as more information arrives.

“We’re confident the levees themselves are in good shape. The big focus is height,” spokesperson Ricky Boyett said.

Carnival Cruise Line said it rerouted a cruise ship headed to New Orleans because of the potential tropical storm brewing in the Gulf. The Miami-based company said the more than 3,700-passenger Carnival Valor was sent to Mobile, Ala., in the interest of safety.

Welcome Carnival Valor to Mobile! She was displaced from New Orleans due to the storm. We will take good care of her and her passengers while she is here. <a href=”https://t.co/fOrze52GPY”>pic.twitter.com/fOrze52GPY</a>